Method for epitaxial growth on a substrate

ABSTRACT

The invention concerns a method for epitaxial growth of a material on a first solid material from a material melting on the material, characterized in that it comprises: a step of growth of the first material on the substrate, made of a second material; a step whereby crystalline tips of the first material are made to grow from the contact surface between the first material and the melting material; a step which consists in causing crystals to grow laterally from the crystalline tips in a plane parallel to that of the free surface of the melting material.

The invention relates to the field of processes for thin-film depositionand for crystalline growth of materials on a substrate. The inventionalso relates to a reactor for implementing this process.

For example, it may be a process for the growth of binary compounds.Certain binary compounds do not exist in the liquid state and largecrystals of these compounds, allowing homoepitaxial growth, are notavailable either. This is especially the case for silicon carbide (SiC)and aluminum nitride (AlN).

For SiC in particular, crystals are obtained by the Acheson method andthen these serve as seeds for growing them by the Lely method. Thecrystals thus obtained are of very good crystalline quality buttypically their dimensions are of the order of one centimeter. They aretoo small for industrial exploitation, a growth method capable ofgrowing them up to 5 to 10 cm is therefore necessary. The so-calledmodified Lely method is currently the only industrial method forproducing SiC as 6H or 4H polytypes. It consists in subliming aparticulate SiC filler at 2300° C. and in condensing it on a seed placedabove it at 2100° C. It is not without drawbacks, most particularlybecause of the temperature at which the growth has to be carried out:2300° C. The equipment for raising to these temperatures is veryexpensive and the difficulties encountered in order to increase the sizeof the crystals are very considerable. Moreover, the crystals obtainedby this method have microchannels deleterious to the production of largepower components.

The crystalline growth of SiC crystals by high-temperature chemicalvapor deposition (CVD) and growth in liquid phase give high growth ratesbut do not make it possible to grow, laterally, i.e. mainly in the planeof the deposition, crystals whose dimensions in this plane aresatisfactory.

A “low-temperature” CVD method also exists for the growth of SiC, whichmakes it possible to grow SiC on silicon substrates of very largedimensions, but the quality of the resulting layers is highlyinsufficient for the fabrication of electronic components because of thepresence of a high density of dislocations due to the crystal latticemismatch between the layer and the substrate.

The situation in the case of AlN is even less favorable since nosupplier of crystals of this material exists.

One objective of the present invention is to provide a process and areactor making it possible to improve the crystalline quality ofcrystals obtained by growth from a liquid phase on a substrate.

This objective is achieved by virtue of the process according to theinvention, which is a process for the crystalline growth of a material,on a solid first material, from a molten material on the solid firstmaterial, characterized in that it comprises:

a step (a) of growing the first material (100) on a substrate (10)consisting of a second material (200),

a step (d, d′) in which crystalline tips of the first material (100) aregrown from the interface between the first material (100) and the moltenmaterial,

a step (f, f′) consisting in growing, laterally, in a plane mainlyparallel to that of the free surface of the molten material, crystalsfrom the crystalline tips.

This is because growth via tips allows the generally high dislocationdensity to be reduced on account of the fact that the first material hasitself many dislocations, for example because of a lattice mismatchbetween the first material and the substrate on which the first materialis grown heteroepitaxially, whereas toward the end of the tips, whichare in the liquid, on the opposite side from the surface of the firstmaterial, stress relaxation occurs, which results in a slight decreasein the number of dislocations, but even for a constant dislocationdensity, because of the small surface area of each tip, the latter onlyhas a few dislocations.

Advantageously, the process according to the invention comprises a stepconsisting in reversing the direction of the temperature gradient.

Thus, when these tips have reached about ten micrometers in height, areversal of the temperature gradient causes lateral growth from the topof these tips. The dislocations, which were very numerous at the surfaceof the first material, are few in number at the top of the tips and veryrare in the crystals which have grown laterally. These crystals areperfectly oriented with respect to one another and coalesce to form asingle monocrystal of very high crystalline quality when the thicknessbecomes sufficiently great. The maximum diameter of the monocrystal isrelated to the maximum diameter of the starting substrate, for example300 millimeters in the case of SiC/Si.

The invention also relates to a crystalline growth reactor forimplementing the process according to the invention, characterized inthat it comprises heating means making it possible to generate atemperature gradient perpendicular to the free surface of the moltenmaterial.

This is because the presence of a temperature gradient makes it possibleto obtain growth via tips which extends in the direction of the gradientrather than two-dimensional growth parallel to the plane of the firstmaterial.

Further benefits, objects and advantages of the invention will appear onreading the detailed description which follows.

The invention will also be more clearly understood by referring to thedrawings, in which:

FIG. 1 is a schematic illustration of various steps of one example ofhow to implement the process according to the invention;

FIG. 2 is a schematic illustration of the various steps of anotherexample of how to implement a process according to the invention;

FIG. 3 is a schematic illustration, in longitudinal mid-section, of oneexample of a reactor according to the present invention;

FIG. 4 is an exploded perspective illustration of the arrangement of thefirst heating means and of the duct, these being used in theconstruction of the reactor in FIG. 3;

FIG. 5 is a front top view of a component allowing the duct to be heldin place inside the chamber of the reactor illustrated in FIG. 3;

FIG. 6 is a schematic illustration, in longitudinal mid-section ofanother example of a reactor according to the present invention; and

FIG. 7 is a schematic illustration, in longitudinal mid-section, of yetanother example of a reactor according to the present invention.

According to a first example of how to implement the process accordingto the invention, illustrated in FIG. 1, this comprises:

a step (a) of growing a first material 100 on a substrate, consisting ofa second material 200 (FIG. 1A);

a step (b) consisting in placing the substrate, with the first material100 beneath the second material 200, horizontally in a crucible 300(FIG. 1B);

a step (c) consisting in taking the second material 200 to the meltingpoint, in a stream of high-pressure inert gas, while keeping the firstmaterial 100 in the solid state, the second material, then in the moltenstate, corresponding to the reference 600;

a step (d) consisting in establishing a temperature gradientperpendicular to the free surface of the molten material, so that theinterface between the first material 100 and the molten second material600 is at a higher temperature than the temperature of the free surfaceof the molten second material 600, and in adding to the stream of inertgas sweeping the surface of the molten second material 600 a precursorgas, at least one first atomic species of which participates, with atleast one second atomic participates coming from the molten secondmaterial 600, in the growth of a fourth material 500, this growth takingplace via tips of the fourth material 500, in crystalline continuitywith the first material 100 (FIG. 1C) from the interface between thefirst material 100 and the molten second material 600;

a step (e) consisting in reversing the direction of the temperaturegradient; and

a step (f) consisting in growing, laterally, in a plane mainly parallelto the free surface of the molten second material 600, crystals from thegrowth seeds that the tips constitute (FIG. 1D).

During step (a), a monocrystalline thin layer of the first material 100is deposited on a substrate of a second material 200 by a conventionaldeposition method known to those skilled in the art, for examplechemical vapor deposition.

In the case of the example described here, the substrate consisting ofthe second material 200 is monocrystalline silicon and the firstmaterial 100 is silicon carbide. The monocrystalline layer of siliconcarbide obtained has a high dislocation density because of the mismatchin the crystal parameters between silicon and silicon carbide.

In step (b) the substrate and the layer deposited on top are placed,with the layer under the substrate, horizontally in a special reactorhaving a controlled vertical temperature gradient and no horizontalgradient. The height of the crucible 300 is such that, during melting ofthe silicon substrate, the liquid does not spill over the edge of thecrucible 300. This condition makes it possible to limit liquid leaksshould the vertical edges of the layer of the first material 100 break,this material being both crucible and growth seed.

After the usual operations of starting up the reactor, an inert carriergas (for example, argon) is introduced into the reactor, preferably at apressure equal to atmospheric pressure or higher, in order to limit thephysical or reactive evaporation of the liquid constituting the moltensecond material 600, at a flow rate high enough to ensure an almostuniform precursor gas concentration over the entire substrate.

During step (c), the temperature is raised above the melting point ofthe second material 200, in this case silicon, ensuring that thetemperature at the free surface of the molten second material 600 isless than that of the interface between the first material 100 and themolten second material 600.

The thickness of the molten second material 600 above the first material100 is advantageously of the order of one hundred or a few hundredmicrometers, or even several millimeters.

During step (d), a precursor gas (for example, propane in the case ofSiC) is mixed with the carrier gas. The precursor gas decomposes at thesurface of the molten second material 600 and the first atomic speciesthat it provides, in this case carbon, diffuses to the interface betweenthe crystalline tips and the molten second material 600 (Si) in order toparticipate in the growth of the fourth material 500, in this case thesame as the first material 100, i.e. silicon carbide. The othercomponents of the precursor gas are removed by the carrier gas to theoutlet of the reactor.

During step (d), there is growth of the crystalline tips of the fourthmaterial 500 on the layer of the first material 100, in the moltensecond material 600. The upper limit of the partial pressure of theprecursor gas, which must not be reached, is that which would cause theformation of a continuous layer of the fourth material 500 on thesurface of the molten second material 600, which would have the effectof instantly stopping any growth. This limiting partial pressure dependson the temperature of the molten second material 600—it is typically1000 pascals.

Under the conditions defined above, the crystalline tips are separatedand distributed fairly uniformly.

Step (e) is started when the crystalline tips have reached a height ofapproximately 10 micrometers. It consists in reversing the direction ofthe temperature gradient, i.e. the free surface of the molten secondmaterial 600 is raised to a temperature above that of the interfacebetween the molten second material 600 and the first material 100, allthe other parameters remaining identical. This causes a lateral growthof the fourth material 500, in this case SiC, from the top of thecrystalline tips, which is continued during step (f).

Step (f) is continued until the crystals coalesce into a thickmonocrystalline layer.

A complete layer 700 of fourth material (SiC) is finally obtained by thecoalescence of all the microcrystals which have grown laterally from thetops of the crystalline tips.

In order to obtain a thicker layer, after the molten second material 600has been exhausted, it is possible to cool the reactor and carry out astep (g) consisting in putting another charge of the second material 200on the fourth material 500 in order to continue its growth in thickness.Thus, for the example described here, a charge of silicon is depositedon the layer 700. The growth is then repeated by heating the moltensecond material 600 and by sweeping its surface as in step (f), i.e.without passing via a new step of forming the crystalline tips.

The typical growth rate of the fourth material 500 thus obtained isseveral tens of micrometers per hour.

According to a second example of how to implement the process accordingto the invention, shown in FIG. 2, this comprises:

a step (a′) equivalent to step (a) already described (FIG. 2A);

a step (b′) consisting in placing the substrate horizontally in acrucible 300, with the first material 100 above the second material 200and a third material 400 on the first (FIG. 2B);

a step (c′) consisting in melting the third material 400 while keepingthe first material 100 and the second material 200 in the solid state;and

steps (d′), (e′), (f′) and, optionally, (g′), equivalent to steps (d),(e), (f) and (g) described above, respectively.

In the case of the growth of aluminum nitride AlN by the processaccording to the invention, the first material 100 and the fourthmaterial 500 are aluminum nitride, the second material 200 is sapphireor else silicon carbide and the third material 400 is aluminum (Al).

Thus, aluminum nitride is deposited on sapphire during step (a′). Instep (b′), the sapphire substrate is placed in the crucible 300 with thealuminum nitride on top. Aluminum is heated to the liquid state on thealuminum nitride during step (c′).

Ammonia or nitrogen are used as precursor gases as a mixture with acarrier gas, in order to deliver the nitrogen, as first atomic species,during step (d′). The rest of the process is equivalent to that alreadydescribed.

The present invention makes it possible to produce SiC wafers withoutany microchannels, for example as 3C and 6H polytypes, of largediameters (up to 200 mm and more), at a temperature of 1500° C. insteadof 2300° C., in a reactor which is inexpensive in terms of investmentand in operating cost.

The process according to the invention, illustrated here with SiC andAlN, may be employed for the growth of other binary compounds, as wellas ternary compounds, etc.

A nonlimiting example of a reactor according to the invention isillustrated in FIG. 3. This reactor 1 comprises a chamber 2 consistingof a tube 3, a first closure plate 4, located at one of the ends of thistube 3, and an outlet cross 5 located at the opposite end of the tube 3with respect to the first closure plate 4. The entire reactor 1 issealed and can possibly withstand a pressure of a few MPa. The sealingof the reactor 1 is provided by seals 32, 33.

The outlet cross 5 may be replaced by a “T”-shaped element.

The axis of the tube 3 is in the horizontal. Placed inside the tube 3,coaxial with it, is a duct 6. Placed outside the tube 3 are coolingmeans 11 capable of cooling the tube 3. The tube 3 is advantageously acylinder made of stainless steel.

The cross 5 is preferably fixed since one of its outlets is connected tothe pumping system.

The outlet cross 5 has a lower orifice and an upper orifice, these beingradially opposed in the vertical direction. The lower orifice of thisoutlet cross 5 emerges either on a pump and a pressure regulator for lowpressures or on a pressure-release valve for a pressure greater thanatmospheric pressure, so as to discharge the gases at constant pressure.These apparatuses are not illustrated in FIG. 3. The upper orifice ofthe outlet cross 5 is hermetically closed off by a second closure plate26. The outlet cross 5 also possesses an orifice longitudinally opposedto the tube 3. This orifice may optionally be provided with a rotatingpassage. In the embodiment presented here, this orifice is closed off bya third closure plate 27 perpendicular to the axis of the tube 3. Thethird closure plate 27 may optionally be equipped with a window or witha moveable mirror for optical measurements inside the duct 6. This thirdclosure plate 27 includes a hermetically sealed port 28 allowingsubstrates 10 to be introduced into or extracted from the reactor 1. Thethird closure plate 27 also includes guides 30, 31. These guides 30, 31are perpendicular to the plane of the closure plate 27 and are solidlyfastened to it. These guides 30, 31 serve for horizontally guiding amanipulator, not illustrated in the figures. The third closure plate 27also includes passages for first current leads 22, 23. Those parts ofthe first current leads 22, 23 which are located toward the inside ofthe chamber 2 are provided with connectors 24, 25.

A duct 6 is positioned and held in place in the tube 3 by virtue ofmeans 35 for fastening a duct 6 to the first closure plate 4. Thus, aduct 6 is held in place so as to be free of any contact with the tube 3.This makes it possible to limit the thermal conduction losses and toavoid thermal stresses.

As illustrated in FIG. 4, the duct 6 is in the form of a tube with arectangular cross section, having a narrowing 36 at one end of it. Thisduct 6 comprises two plates for forming lower 37 and upper 38 walls. Thelower 37 and upper 38 walls of the duct 6 are horizontal and parallel tothe plane of the substrate 10 in the position that it occupies duringthe deposition. Side walls 39, 40 join the longitudinal edges of thelower 37 and upper 38 walls in order to close the duct 6 longitudinally.That end of the duct 6 which is located on the same side as thenarrowing 36 has a square cross section. It is provided with a supportplate 41 perpendicular to the longitudinal axis of the duct 6. Thissupport plate 41 has an opening facing the mouth of the duct 6, locatedon the same side as the narrowing 36. The support plate 41 also hasholes in order to allow the duct 6 to be fastened to the first closureplate 4, using the fastening means 35. When the duct 6 is fastened tothe first closure plate 4, the mouth of the duct 6, located on the sameside as the narrowing 36, and the opening in the support plate 41 liefacing a gas inlet 7. The duct 6 is connected in a sealed manner to thefirst closure plate 4 at the gas inlet 7. The joint between the duct 6and the first closure plate 4 is sealed by tightening a graphite sealfor example, using the fastening means 35.

The gas inlet 7 serves for supplying the reactor 1 with carrier andprecursor gases. The first closure plate 4 is also provided with a gaspassage 44 which is offset with respect to the axis of symmetryperpendicular to the plane of the disk formed by the first closure plate4 and which emerges between the duct 6 and the wall of the tube 3. Thegas passage 44 also allows the introduction of gas into the reactor 1.The gas passage 44 allows the flow of a gas which is inert with respectto all of the materials included in the reactor 1 and with respect tothe material to be deposited and to the gases flowing in the duct 6,this inert gas preventing any return of the gases resulting from theprocess toward the heating parts external to the duct 6.

Preferably, the duct 6 is in a material which, all at the same time, isa good heat conductor, is a good electrical insulator, is highlyrefractory, is chemically very stable and has a low vapor pressure atthe operating temperatures, although, optionally, a prior coating of thematerial intended to be deposited on a substrate 10 in this reactor 1can be deposited on the internal face of the walls 37, 38, 39, 40 of theduct 6 so as to minimize the diffusion of any outgasing substancesduring the normal operation of the reactor 1.

Also advantageously, this material has good mechanical strength in orderto allow the walls 37, 38, 39, 40 of the duct 6 to have a smallthickness. The small thickness of these walls 37, 38, 39, 40 makes itpossible to minimize the thermal conduction losses.

The mechanical strength of the material of the duct is also important inbeing able to support this duct 6 only by its end located on the sameside as the narrowing 36 and the support plate 41.

The constituent material of the duct 6 is advantageously boron nitridefor use at temperatures of less than 1200° C., or at higher temperaturesif the presence of a high nitrogen concentration does not impair theexpected quality of the material produced.

For higher temperatures, the duct 6 may be made of graphite. In onecase, just as in the other, the duct 6 may be internally lined in thehottest parts with a secondary duct made of a refractory material, forexample a refractory metal, which is inert with respect to the gasesflowing in the duct 6 and is noncontaminating with respect to thematerial deposited. The duct 6, whether made of graphite or boronnitride, may be produced either by pyrolitic deposition, or byassembling and/or adhesively bonding the various constituent plates ofthe walls 37, 38, 39, 40 and the support plate 41. This secondary duct,when there is one, advantageously lines the inside of the duct 6 in acontinuous manner, that is to say that, if it consists of plates, theseare contiguous and there are no holes in these plates. The secondaryduct is, for example, made of tungsten, tantalum, molybdenum, graphiteor boron nitride.

By way of example, the thickness of the walls of the duct 6 is less thanor equal to approximately 1 mm; the internal height of the duct 6 ispreferably less than 30 mm; the width of the duct 6 is equal to thewidth of a substrate 10 or to the sum of the widths of the substrates 10which are treated during the same deposition, plus approximately 1 cmbetween the substrate or substrates 10 and the walls 39 and 40.

That part of the duct 6 corresponding to the narrowing 36 corresponds toabout ⅕ of the total length of the duct 6. The length of the part with aconstant cross section of the duct 6 is equal to approximately fivetimes the diameter or the length of the largest substrate 10 which it isdesired to use or five times the sum of the diameters or lengths of thesubstrates 10 on which a deposition may be carried out during the sameoperation. That part of the duct 6 which extends over a lengthcorresponding to the diameter or to the length of a substrate, or to thesum of the lengths or the diameters of the substrates, is calledhereafter the deposition zone.

Advantageously, the reactor 1 is provided with first 8 and second 9heating means placed near the deposition zone and located on either sideof the plane of the substrate 10.

Advantageously, these first 8 and second 9 heating means consist of bareresistive elements, i.e. the constituent material of the first 8 andsecond 9 heating means is in direct contact with the gas flowing betweenthe duct 6 and the tube 3.

Each resistive element corresponding respectively to the first 8 or tothe second 9 heating means consists of a band, i.e. a rigid plateelement, or of a strip, which is placed flat and parallel to the lower37 and upper 38 walls of the duct 6 (FIG. 4). This strip or band has asuitable geometry so that, in the deposition zone, the deviations fromthe mean temperature on that surface of the substrate 10 which isintended for the deposition are minimized. More preferably, thesedeviations are less than 3° C. Preferably, each resistive element has adimension in the direction parallel to the width of the duct 6 which isapproximately equal to this width. The dimension of each resistiveelement in the direction parallel to the length of the duct 6 isapproximately equal to twice the length of the deposition zone. This inorder to optimize the uniformity of the temperature field in thedeposition zone. Preferably, each band or strip of a resistive elementconsists of bands parallel to one another in the longitudinal directionof the tube 3, these bands joining each other in pairs alternately atone or other of their ends so as to form a zigzag geometry. Othergeometries can be envisaged, such as spiral geometries.

Each resistive element may have a longitudinal resistance profile, forexample obtained by varying its thickness, the profile being suitablefor favoring the formation of a controlled temperature profile in thedeposition zone.

Each resistive element has a high filling coefficient in the depositionzone so that their temperature remains as little as possible above thedesired local temperature.

The space between the bands of the resistive elements is sufficient toavoid an arc or a short circuit, but is also small enough to maintainacceptable homogeneity in the temperature field and for it not to benecessary for its temperature to be much higher than that of the ductwhich is itself that at which the deposition takes place. Preferably,the first 8 and second 9 heating means are supplied with a voltage ofless than or equal to 240 volts and more preferably still of less thanor equal to 100, 110 or 120 volts.

Optionally, the first heating means 8 and second heating means 9 eachconsist of several resistive elements of the type of those describedabove.

Advantageously, the resistive elements are made in an electricallyconducting and refractory material having a very low vapor pressure atthe operating temperatures. This material may, for example, be graphite,a metal such as tantalum or tungsten, or else a refractory alloy, etc.Preferably, it is high-purity graphite.

The first 8 and second 9 heating means are supplied with currentindependently of each other so as to be able to be raised to differenttemperatures. It is also possible to generate a temperature gradientperpendicular to the plane of the substrate 10. This gradient may have apositive, negative or zero value, by independently controlling theelectrical power applied to one of the first 8 or second 9 heatingmeans.

The first 8 or second 9 heating means may be applied, outside the duct6, in the region of the deposition zone, so that they are in contactwith the lower 37 and upper 38 walls, respectively. However, accordingto a variant, these means are each positioned, outside the duct 6, so asto be a distance of 1 to 3 mm from one of the lower 37 or upper 38walls, respectively. The first 8 and second 9 heating means are eachpressed against the lower 37 and upper 38 walls by retention plates 12,13 which are electrically insulating and thermally conducting. If theduct 6 is not an electrically insulating material, it is necessary toput, between the duct 6 and the first 8 and second 9 heating means, anelectrically insulating intermediate material in order to avoid anyelectrical contact, especially in the hot zone, if very hightemperatures have to be achieved.

These retention plates 12, 13 may be made of boron nitride and have athickness of approximately 1 mm, or even less. It is also particularlyadvantageous to confine the retention plates 12, 13 to the coolest endsof the elements of the duct 6 so as to prevent decomposition of theboron nitride and formation of nitrogen. Boron nitride sheaths designedto house thermocouples 51 may be cemented to the retention plates 12,13, but they may also be free above the first 8 and second 9 heatingmeans. These thermocouples 51 (not shown in FIGS. 3 to 5) are used formeasuring the temperature of the duct 6, to regulate it and to controlthe homogeneity of the duct in the deposition zone. They can be used fortemperatures of less than 1700° C. (for temperatures greater than 1700°C., the temperature must be measured by optical pyrometry or bythermocouples without any contacts). The hot junction of thesethermocouples 51 is located outside the duct 6 as close as possible tothe first 8 and second 9 heating means.

When the duct 6 is made of graphite, that is to say when it isconducting, the first 8 and second 9 heating means may be made of rigidgraphite. They are then electrically isolated from the duct 6 byspacers, for example made of boron nitride, which separate them from theduct 6 by a few millimeters. These spacers may be fastened to the endsof the first 8 and second 9 heating means and therefore may not beheated excessively. One or more sheaths, made of graphite or of boronnitride, may be fastened to the faces of the duct 6 in order to housethermocouples which are themselves insulated in refractory andelectrically insulating sheaths.

As shown in FIG. 4, the first 8 and second 9 heating means, as well asthe retention plates 12, 13, are held together against the duct 6 bymeans of cradles 16, 17. Each cradle 16, 17 consists of two half-diskswhich are parallel to each other and connected together by rods whichare perpendicular to them. The diameter of the disks, consisting of twohalf-disks, is slightly less than the internal diameter of the tube 3.The straight edge of the two half-disks is in a horizontal plane. Eachstraight edge of each half-disk includes notches capable of receiving aretention plate 12 or 13, the first 8 or second 9 heating means as wellas half the height of the duct 6. The resistive elements of the first 8and second 9 heating means are kept isolated from the duct 6 by thecradles 16, 17.

The dimension of these cradles 16, 17 in the direction parallel to thelongitudinal axis of the duct 6 corresponds approximately to the lengthof the first 8 or second 9 heating means in that direction.

These cradles 16, 17 are placed approximately in the middle of the duct6, considered along its longitudinal direction.

Advantageously, the half-disks of the cradles 16, 17 are in contact withthe duct 6 in the cool parts of the latter.

Heat shields 14, 15 are placed on either side of the first 8 and second9 heating means, outside the latter. More specifically, heat shields 15are located between the internal wall of the tube 3 and the curvilinearpart of the half-disks making up the cradles 16, 17. They extend belowthe internal face of the tube 3, but without contacting the latter,concentrically around the heating zone. Other heat shields 14 are placedbetween the retention plates 12, 13 and the previous heat shields 15.These heat shields 14, 15 are composed of two or three thin sheets ofpolished, reflecting and refractory metal such as tantalum, molybdenum,etc. The outermost heat shield 14 or 15 is at the closest point a fewmillimeters from the internal wall of the tube 3. This longitudinalconfiguration, with the first 8 and second 19 heating means inside thetube 3, in contact with the duct 6, and two or three heat shields 14,15, greatly limits the radiation losses which would otherwise be veryconsiderable at high temperatures, such as those required for thedeposition of silicon carbide.

The half-disks of the cradles 16, 17 are made in an electrically andthermally insulating material. Thus, the heat shields 14, 15 areelectrically and thermally insulated from each other and from theheating means 8, 9.

The assembly consisting of the duct 6, the first 8 and second 9 heatingmeans, the retention plates 12, 13, the cradles 16, 17 which hold allthese elements together, together with the heat shields 14, 15, isplaced in the tube 3. This assembly limits the flow of gas outside thehot part of the duct and thus helps to limit the heat losses.

Advantageously, two disks 18, 19 are placed, perpendicular to the axisof the tube 3, between the cradles 16, 17 and the outlet cross 5.

As shown in FIG. 5, these disks 18, 19 are provided with a rectangularcentral opening, the area of which corresponds approximately to thecross section of the duct 6, so as to be able to slip these disks 18, 19onto this duct 6. These disks 18, 19 also have holes peripherals to thecentral opening and intended for passage of second current leads 20, 21and thermocouple wires 51. One 19 of these disks 18, 19 is placed in theoutlet cross 5. The other 18 of these disks 18, 19 is placed between thedisk 19 and the cradles 16, 17. The purpose of these disks 18, 19 is tohold together the duct 6, the second current leads 20, 21 and the wiresof the thermocouples 51, as well as to limit the gas exchange betweenthe inside of the duct 6 and the space lying between the duct 6 and thetube 3. However, the disks 18, 19 must allow passage of the gases comingfrom the outlet of the duct 6, between the internal space of the duct 6and the space lying between the duct 6 and the tube 3, so that thepressure is balanced on either side of the walls 37, 38, 39, 40. By thusbalancing the pressure on either side of the walls 37, 38, 39, 40, it ispossible to make the latter with a small thickness.

The pairs of second current leads 20, 21 are connected to the firstcurrent leads 22, 23 by means of connectors 24, 25. The thermocouples 51are also connected to the outside of the chamber 2 via connectorslocated in the chamber 2.

The disks 18, 19 may be made of an electrically and thermallyinsulating, but not necessarily highly refractory material.

The hermetically sealed port 28 covers an opening whose width isapproximately equal to that of the duct. This opening is located on theaxis of the duct 6. It allows the substrates 10 to be inserted andremoved. An inlet airlock is possibly connected to the third closureplate 27 in order to prevent, during the operations of inserting andremoving the substrates 10, from venting the reactor 1 to atmosphereagain.

The substrates 10 are advantageously inserted into the reactor 1 bymeans of a substrate holder 29. Advantageously, the substrate holder 29is made of a material which is a good thermal conductor so that it haslittle thermal inertia. Preferably, this substrate holder 29 is made ofboron nitride, but it may also be made of graphite for example. Thesubstrate holder 29 is inserted into the reactor 1 by a grippermanipulator which slides along the guides 30, 31. This manipulatorconsists of a thin rigid tube coaxial with the axis of the duct 6, of along rod threaded on the inside of this tube and fastened, on thereactor 1 side, to two symmetrical gripper elements which are hingedabout a vertical hinge, the outer end of the threaded rod being screwedby a freely rotating captive nut. By screwing the nut, the threaded rodretracts and the gripper is thermally clamped onto a vertical part ofthe substrate holder 29. The manipulator can then be moved along theguides 30, 31 in order to insert or remove the substrate holder 29. Acam on the manipulator may be provided in order to allow the gripper tobe raised, when the latter has just seized the substrate holder 29, inits position inside the duct 6, so that the latter does not rub againstthe internal face of the wall 37.

Before commissioning the reactor 1, a coating of the predominant productto which the reactor 1 is dedicated is deposited in the duct 6, withneither the substrate 10 nor the substrate holder 29, after a thoroughdegassing step, at a temperature greater than the usual depositiontemperature, and a thorough purge with the carrier gas. This step may befollowed by a similar deposition on the substrate carrier 29, withoutthe substrate 10. The reactor is then ready to be used.

Variants of the process and the reactor according to the invention arepossible.

In another embodiment of the reactor (FIG. 6), the current leads 22, 23and the thermocouple outputs may advantageously be located on the sameside as the gas inlet 7. The loading and unloading of the substrates 10may then be accomplished by disconnecting the body of the reactor 3 fromthe cross 5. It is then advantageous to install a rotary substrateholder 29 actuated via a sealed rotary passage and driven so as to passaxially through the closure plate 27. This arrangement is particularlyuseful in steps (a) and (a′) of the processes described above.

Resistive first 8 and second 9 heating means were described above. Thistype of heating means makes it possible to reach temperatures greaterthan 1750° C. with a low investment in terms of materials and a lowerenergy consumption than with the processes and the reactors of the priorart.

For example, in order to reach the melting point (1410° C.) of a siliconwafer 50 mm in diameter, in a stream of hydrogen of 8 liters per minute,at a pressure of 5×10³ pascals, a power of 3 kW is sufficient. Likewise,in this environment, in order to raise the temperature from 500° C. to1400° C., at a rate of 100° C. per second, a 7 kW power line is alsosufficient.

However, other types of heating means 8, 9 may be envisaged, even ifthey seem less advantageous, such as induction heating means, heatingmeans in which the first 8 and second 9 heating means form only a singledevice placed all around the duct 6, etc.

FIG. 6 shows another embodiment of the reactor 1 according to theinvention. In this embodiment, the reactor 1 comprises a chamber 2consisting of two concentric stainless steel tubes 3, 103, the commonaxis of revolution of which is horizontal. A coolant flows in the spacebetween the two walls of these tubes 3, 103.

An antisplash nozzle 50 is mounted on the axis of the gas inlet 7 so asto be conducive to achieving good gas velocity uniformity. The gaspassage 44 may also optionally be provided with an antisplash nozzle.

A mechanism 60, driven by a shaft 61 which passes through a gastightpenetration 62, and a sliding coupling 63, allows the substrate 10 to berotated so as to ensure greater uniformity of the deposition.

All the electrical and fluid connections to the third closure plate 27and the first closure plate 4 are sufficiently long and flexible to beable to move them over approximately twice the length of the duct 6.Advantageously, the connections may also be made only to the firstclosure plate 4.

The first closure plate 4 is fastened to a carriage comprising avertical support 64 and a horizontal support 65.

The horizontal support 65 may be moved parallel to the axis of the tube3 over a running track, not shown. In order to mount the assemblycomprising the duct 6 and its equipment, the first closure plate 4 isopened, the tube 3 remaining fastened to the cross 5.

To load or unload a substrate 10 there is the choice between opening thethird closure plate 27 or separating the tube 3 from the cross 5.

The substrates 10 are inserted and held in place in the deposition zoneby a graphite substrate holder 29 which may be raised a few degrees, onthe downstream side with respect to the gas flow, so as to offer agreater area of projection on a vertical plane, in the duct 6. Thesubstrate holder consists, for example, of a disk with a rim.Advantageously, the rim has a height greater than the height of thesubstrate 10. The substrate holder 29 can rotate the substrate 10 whichit supports, so as to ensure better deposition uniformity.Advantageously, this is achieved by virtue of a mechanical transmissionconsisting of a bevel gear having a horizontal axis and fastened to theshaft 61, the latter being rotated by a motor external to the reactor 1,at a variable speed, providing a substrate rotation speed possiblyranging up to 10 revolutions per second.

According to an advantageous variant, not shown, of the reactoraccording to the present invention, the reactor has first 8 and secondheating means which are offset one with respect to the other in thelongitudinal direction of the duct 6. This also makes it possible forthe temperature distribution to be made uniform over the entire surfacearea of the substrate, favoring the formation of a plateau in thelongitudinal temperature profile.

According to another advantageous variant, the center of the substrate10 on the substrate holder 29 is shifted toward the downstream directionof the gas flow, in the zone of the first 8 and second 9 heating means,without nevertheless the substrate 10 leaving this zone.

According to yet another variant, illustrated in FIG. 7, the secondaryduct consists of removable plates 70 which may be easily inserted andremoved by sliding in grooves, not shown, in the duct 6. These plates 70are useful for protecting the duct 6 from the deposition away from thesubstrate(s) 10. They are easily maintained and advantageously made ofgraphite, boron nitride or another refractory material compatible withthe temperature of the process and with the ambient medium.

According to another advantageous variant, also shown in FIG. 7, thetemperature may be measured by optical pyrometer fibers 71 located insheaths fastened to the duct 6 and between the duct 6 and the first 8and second 9 heating means, rather than by thermocouples 51, so as toincrease the lifetime of the means for measuring the temperature.

The process according to the invention makes it possible to achieve theaforementioned advantages while maintaining a level of impurities in thelayers obtained which is equivalent to those of the layers obtained bymeans of the processes and the reactors of the prior art.

A process and a reactor according to the invention are particularly wellsuited to the growth of silicon carbide or aluminum nitride layers onsubstrates 10.

What is claimed is:
 1. A process for growing crystals of a moltenmaterial on a solid first material comprising: growing the firstmaterial on a substrate consisting of a second material, melting thesecond material, thereby forming the molten material, while keeping thefirst material in a solid state, or melting a third material, therebyforming the molten material, while keeping the first material and secondmaterial in a solid state, growing crystalline tips of the firstmaterial from an interface between the first material and the moltenmaterial, and growing, laterally, in a plane mainly parallel to that ofthe free surface of the molten material, crystals from the crystallinetips.
 2. The process according to claim 1, further comprising imposing atemperature gradient in a direction perpendicular to the free surface ofthe molten material.
 3. The process according to claim 2, furthercomprising reversing the sign of the gradient after the tips have grownto a desired height.
 4. The process according to claim 1, furthercomprising placing the substrate, with the first material beneath thesecond material, horizontally in a crucible, before melting the secondmaterial.
 5. The process according to claim 1, further comprisingplacing the substrate horizontally in a crucible, with the firstmaterial above the second material and a third material above the firstmaterial.
 6. The process according to claim 1, further comprisingsweeping the molten material with a precursor gas, wherein at least onefirst atomic species of the gas participates with at least one secondatomic species from the molten material in the growth of crystallinetips of a fourth material in crystalline continuity with the firstmaterial, wherein the crystalline tips of the fourth material are grownwhen the interface between the first material and the molten material isat a higher temperature then the temperature of the free surface of themolten material.
 7. The process according to claim 6, wherein the fourthmaterial is a binary compound.
 8. The process according to claim 7,wherein the binary compound is silicon carbide.
 9. The process accordingto claim 7, wherein the binary compound is aluminum nitride.
 10. Theprocess according to claim 1, wherein the growing of the crystalslaterally from the crystalline tips, is carried out by reversing thesign of a temperature gradient.
 11. The process according to claim 6,further comprising placing a charge of the second material or of thethird material on the fourth material in order to continue its growth inthickness.
 12. The process according to claim 1, wherein the growing ofthe first material on a substrate is carried out by chemical vapordeposition.
 13. The process according to claim 6, wherein the growth ofthe fourth material, which is in crystalline continuity with the firstmaterial, proceeds from the crystalline tips situated at the interfacebetween the first material and the molten material.
 14. The processaccording to claim 1, wherein the temperature gradient is reversed whenthe crystalline tips have reached a height of approximately 10micrometers.
 15. The process according to claim 3, wherein the growingof the crystals from the crystalline tips laterally occurs after thereversal of the temperature gradient.